The multi-carrier method OFDM (orthogonal frequency division multiplex) is becoming more widespread as time goes by. It is used for digital radio (digital audio broadcasting or DAB), digital television (digital video broadcasting or DVB) and for local radio networks (high performance local area network or Hiperlan).
OFDM differs from single-carrier methods in that the information flow is transmitted not by way of a single carrier but by way of a number of subcarriers. A frame to be transmitted is subdivided into a number of symbols. Each symbol contains a number of data which are transmitted from the transmitter to the receiver distributed to various subcarriers. A given frequency spacing is maintained between the subcarriers. The orthogonality of the subcarriers is achieved by orthogonally encoded digital modulation. The orthogonality of the subcarriers ensures distinguishability thereof even in the case of spectral overlap.
Not all subcarriers in an OFDM system contain user information. There are so-called pilot subcarriers, also referred to hereinafter as pilot channels, for the transmission of pilot symbols. The pilot symbols contain information which is known from the outset to the receiver. In the transmission of a frame a plurality of pilot signals are transmitted, which are integrated into the frame at usually regularly spaced positions.
Pilot subcarriers are basically used for channel estimation. In that case channel attenuation of the various subcarriers which are also referred to as subchannels is ascertained by means of interpolation of the received values of the pilot subcarriers. The standards “IEEE 802.11a” and “Hiperlan/2” for wireless communication in networks in the 5 GHz frequency range do not permit use of the pilot channels for channel estimation. The pilot channels are here provided in mutually different frequency ranges which involve a spacing of 4.375 MHz. Therefore, only channels with a maximum delay spread of 2/4.375 MHz=0.46 μs can be correctly estimated by interpolation of the pilot channel information. In general terms however the pilot subcarriers cannot be used in the specified standards for channel estimation.
Correct decoding of OFDM signals on the receiver end is delicately dependent on the efficiency of the synchronization unit of the receiver. The synchronization unit is responsible for the detection of incoming frames and for estimating and correcting possible frequency offsets. Frequency offsets produce errors which lead to incorrect decoding of the received signal.
WO 01/20863 A1 describes a method of and an apparatus for correction at the receiver end of a phase error of the received signal in the time domain. There is provided a phase estimation circuit having two phase locked loops. The correction of a phase error in the time domain has the disadvantage of a great delay for the signal. The use of phase locked loops for phase estimation additionally signifies a high level of circuitry complication and expenditure.
Other methods provide for phase estimation and phase correction in the frequency domain. Provided for that purpose is an FFT unit which transforms the received signal by means of a fast Fourier transform or FFT into a frequency spectrum which can be broken down according to subcarriers. In those methods a synchronization unit also decides about the start time from which the incoming signal is subjected to a FFT.
The publication V. Mignone, A. Morello: “CD3-OFDM: A Novel Demodulation Scheme for Fixed and Mobile Receivers” (IEEE Transactions on Communications, Vol. 44, No. 9, September 1996, pages 1144 to 1151) discloses a method and an apparatus in which the Fourier-transformed signal of each subcarrier is subjected to equalization prior to decoding. Equalization is effected by means of the signal received in the preceding cycle. For that purpose that signal is decoded, then recoded in a kind of feedback loop and used for channel estimation of the corresponding subcarrier. The frequency response function ascertained from the channel estimation is used for equalization of the current Fourier-transformed signal by the current signal being divided by the frequency response function ascertained from the preceding signal.
It has been found however that, after the Fourier transform and equalization in the frequency domain and decoding of the symbols phase errors are contained in the equalized signal. That gives rise to errors in the decoding operation, which can result in a data communication crashing.
Conventional methods of eliminating that residual phase shift involve correction of the phase error by means of complex multiplication with a suitable phase factor of the value 1 (phasor). Complicated circuits which are based on numerically controlled oscillators (NCO) are usual for correction of the phase error. Those circuits cause a severe delay in terms of signal decoding.